In well drilling operations, it is conventional to use a downhole fluid motor to rotate the drill bit when conducting special drilling operations such as, for example, directional drilling of boreholes. However, while downhole fluid motors of the progressive cavity type, also known as Moineau or positive displacement motors, have proven to be effective for generating rotational motion at the end of a drill string, there are inherent drawbacks related to the design of such motors. One particular drawback relates to the connection between the motor and the drill bit.
To fully understand the problem, one must understand the basic design of such fluid motors. In particular, fluid motors of the above type comprise a helical rotor positioned within the cavity of a helical stator. Drilling fluid, which is pumped down through the drill string to cool the drill bit and to carry drill cuttings to the surface, is directed down through the annulus between the rotor and stator to cause rotation of the rotor. However, the rotor does not simply rotate about a fixed vertical axis. The rotor rotates while gyrating, or translating along an orbital path. More particularly, the orbital path is not a simple circular orbit but rather an eccentrically orbital path typified by lateral excursions toward and away from the axis of the downhole fluid motor. Such complex gyrational and rotational motion of the rotor tends to be very demanding on the connecting shaft which connects the rotor to the drill bit and converts the gyrational and rotational motion to pure rotation.
Several arrangements have been developed for the design of the connecting shaft which have not been entirely satisfactory. A first arrangement for connecting the fluid motor to the drill bit and transforming the translating and rotating motion of the rotor to the pure rotation of the drill bit comprises a specially manufactured torque shaft. The special torque shaft is made of high strength, high quality steel and is machined to a near mirror surface finish. The high quality finish is necessary because the cyclical bending and flexing of the shaft would quickly cause fatigue cracking of the steel resulting in failure of the shaft. However, the metallurgical and production costs of such torque shafts are substantial and their reliability has been less than satisfactory. Moreover, such shafts have intrinsic design limitations which are particularly unsatisfactory. For example, in a large diameter drill string, the rotor follows a path having more exaggerated lateral excursions which result in increased bending stresses for the torque shaft.
A second arrangement is an articulated or double knuckle connector shaft which includes universal joints at opposite ends thereof. The universal joints obviate any bending stresses such as incurred by the above arrangement and are preferred in the larger diameter drill strings. However, the universal joints include seals which are subject to excessive wear and failure downhole. Also, in drill strings of less than 61/2 inches in diameter, the universal joints must be of such a small size that the load bearing elements in the joints are subject to high shear stress. Therefore, the shaft has less than acceptable maximum allowable loading characteristics. This is a particular drawback for hard rock drilling where impact loading would exceed design limitations of the joints. Accordingly, the double knuckled connector has been less than fully satisfactory.
U.S. Pat. No. 4,679,638 issued Jul. 14, 1987 to Eppink discloses a torque shaft which comprises a flexible rod and a coaxial overload sub surrounding the flexible rod. The flexible rod is similar to the torque shaft in the first arrangement discussed above, however, its maximum torque strength need not be as great since the overload sub will carry a portion of the load. The overload sub includes splines which intermesh but do not normally engage with splines at the outer surface of the rod. When the rod twists beyond a predetermined deflection, the splines engage and the overload sub carries a portion of the load. However, this type of arrangement occupies more space than the above arrangements and the additional space may be critically important for achieving drilling fluid flow rates for drilling in particular types of formations. Moreover, the rod is still subjected to the same fatigue stresses as the torque shaft in the first arrangement above as well as the additional twisting deflections prior to the spline engagement. Thus, the rod may be prone to fatigue cracking.
Accordingly, it is an object of the present invention to provide a connector for a downhole fluid motor which overcomes the above noted disadvantages and drawbacks of the prior art.
It is a more particular object of the present invention to provide a connector for a downhole fluid motor which has high torque strength and stiffness characteristics while accommodating bending flexibility with minimal bending fatigue and failure.